GB2529140A

GB2529140A - Polymer foam production improvement

Info

Publication number: GB2529140A
Application number: GB1412229.5A
Authority: GB
Inventors: Peter Winnicki; Paul Jacobs
Original assignee: Zotefoams PLC
Current assignee: Zotefoams PLC
Priority date: 2014-07-09
Filing date: 2014-07-09
Publication date: 2016-02-17
Anticipated expiration: 2034-07-09
Also published as: GB2529140B; GB201412229D0

Abstract

A coating formulation comprising at least one of a water dispersion medium, a particulate material which is either talc or magnesium carbonate; and an adhesion promoter selected from sodium carboxymethylcellulose, cellulose ether, a styrene acrylic copolymer emulsion or any combination thereof. The coating formulation can be applied to the surface of a polymer artifact, such as polymer foam, to prevent it adhering to the solid substrate on which it is placed during the gas absorption and expansion stages of the nitrogen autoclave foaming process. The coating can be easily applied using commercially available spray coating equipment or brushing and can be easily removed by washing with water.

Description

POLYMER FOAM PRODUCTION IMPROVEMENT

FIELD OF THE INVENTION

S The present invention relates to improvements in or relating to the production of polymer foams made by the nitrogen autoclave process.

BACKGROUND OF THE INVENTION

The nitrogen autoclave process has been used to produce polymer foams commercially for many years and is described in some detail in patents US 3,640,915 and GB 899,389 as well as in books such as Polymeric Foams, edited by ST. Lee and D. Scholz (CRC Press, 2009) and Handbook of Polymer Foams, edited by D. Eaves (Rapra Technology Limited, 2004). The process typically comprises the stages of extrusion and crosslinking of the polymer, gas absorption into the polymer under high pressure and at elevated temperature and, finally, expansion of the polymer, again at elevated temperature, where elevated temperature means a temperature of at least 10°F (i.e. 5.5°C) above the sofiening point of the polymer as taught by Cooper in both US 3,640,915 and GB 899,389. However, in none of the above prior art is the inherent problem addressed of how to prevent the polymer artefact, typically in the form of a thick extruded sheet, sticking to the solid substrate on which it is placed during the gas absorption and expansion stages of the process.

In practice, this problem was addressed by the liberal use of loose talc to cover the surfaces of the polymer artefact and the solid substrate, which would typically take the form of a metal tray. However, the use of copious amounts of loose talc in the production environment proved undesirable for a number of reasons. Firstly, there were housekeeping issues as the talc would coat uncovered surfaces (equipment, floors, pipework, ducting, etc.) with a fine layer of dust that would have to be physically removed by vacuum cleaner or some other means, secondly, there were potential heath issues from the presence of airborne dust in the factory environment and, thirdly, the presence of loose talc inside the autoclaves used for the gas absorption and expansion stages of the process resulted in the abrasion of pipework and valves as a result of talc particles becoming suspended and carried in the gas stream at relatively high velocities.

Therefore, it was recognised that some form of coating that allowed the reduction, or even elimination, of the loose talc used in the process would be desirable. Howeveç the requirements for such a coating have proved to be difficult to achieve in practice.

Firstly, the coating needs to adhere to the surface of the polymer artefact throughout the various stages of the nitrogen autoclave process, including any expansion of the polymer artefact at the end of the gas absorption stage and any thermal expansion or contraction during the various stages of the process. The coating also needs to adhere to the surface of the polymer artefact at reduced temperatures such as those used to reduce gas loss between the gas absorption and expansion stages. Where the period between gas absorption and expansion is prolonged, as, for example, in cases where the gas absorption stage and the expansion stage take place in different locations, the gas containing artefact could potentially be stored at temperatures of -40°C or below.

Secondly, the coating needs to be able to survive the high temperature and pressure environment of the gas absorption stage without any significant degradation and without adversely affecting the surface or internal quality of the final foamed part. This is a particular challenge as both the temperatures and pressures used in the gas absorption stage have increased over the years as new polymer foams and processing equipment have been developed.

Thirdly, but by no means least, the coating should not impair the gas absorption process, as might happen if the coating were to form an impermeable layer on the surface of the polymer.

In addition to the requirements mentioned above, the coating needs to be easy to apply to the surface of the polymer artefact and easily removed at the end of the process, without adversely affecting the surface quality of the final foamed part. Ideally, the coating should be able to be sprayed onto the surface of the artefact using such apparatus as is typically used for spray painting, powder coating or the like. Where the coating is applied in the form of a liquid, the result should be a uniform coating on the surface of the artefact without "beading up", as can sometimes happen when a liquid or suspension is applied to the surface of a polymer artefact with a low surface energy where wettability is an issue. In addition, where the coating is applied in the form of a liquid, it would also be beneficial if the coating could be easily and quickly dried, either within the normal factory environment or at elevated temperature, for example, in a drier or hot air oven. With regard to the removal of the coating, it would be desirable if the coating could be easily and quickly removed without the need for solvents or specialist equipment. to

As the coating is essentially a consumable within the manufacturing process it would be advantageous if the cost of the coating formulation and the method of application and removal were kept to a minimum. It is further desirable to keep the environmental impact to a minimum, particularly with regard to the use of any solvents and the S treatment of any effluent streams generated and, as such, a water-based coating system

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, there is provided a coating formulation for application to a polymer artefact to reduce adhesion of the artefact to a surface, wherein the coating formulation comprises at least one of each of the following constituents: (i) a dispersion medium; (ii) a particulate material; and (iii) an adhesion promoter.

The present invention is a coating that can be applied to the surface of polymer artefacts to reduce, or eliminate, the use of the loose talc that has traditionally been used to prevent the artefacts adhering to a substrate, or to each other, during the gas absorption and expansion stages of the nitrogen autoclave process used to produce polymer foams.

In these stages the coating may be exposed to temperatures in excess of 250°C and pressures in excess of 670 bar.

The coating is preferably in the form of a water-based mixture that can be applied to the S surface of the polymer artefact before the gas absorption stage of the nitrogen autoclave process, using commercially available spray coating and drying equipment, and can be removed from the surface of the polymer artefact after the expansion stage of the nitrogen autoclave process, by washing with water. The solid residues from the coating can then be physically removed from the water used to wash the finished artefact by the tO use of filtration, centrifuges, settling tanks or other equipment without the need for chemical treatment of the water.

The coating mixture comprises, at least, the following components: tS a) Dispersion medium b) Particulate material c) Adhesion promoter Other components, which bring specific attributes to the coating, such as antioxidants or acid scavengers, or facilitate the coating or washing processes, such as wetting agents or anti-foaming agents, may also be contemplated.

As regards the dispersion medium, water is the preferred dispersion medium for the coating mixture, It is non-hazardous, environmentally friendly and results in a coating that can be easily applied with commercially available equipment and dries relatively quickly. In addition, the fact that the coating is water-based also means that it is relatively easy to remove, at the end of the process, by washing with water.

As regards the particulate material, this may be selected from a number of commonly available minerals or fillers that are insoluble in water, for example, talc, mica, expanded graphite, glass microspheres, silica, PTFE (polvtetrafluoroethylene) or calcium carbonate, or combinations of such minerals and fillers. Preferably the particulate material is available in the form of a micronized powder with a lamellar or plate-like structure, is non-abrasive and chemically inert. The particulate material should also be thermally stable at the temperatures used for the gas absorption arid expansion stages of the process. In one prefered embodiment a talc, such as Talc S2 (LKAB Minerals), is used as the particulate material, however, this is not suitable for all polymers, for example polyvinylidene fluoride, and so in a second preferred embodiment a finely ground magnesium carbonate containing mineral, Ultracarb US (LKAB Minerals), is used in place of the talc.

It has also been noted that in some instances it is advantageous to include a second particulate material to improve the suspension of the first particulate material in the dispersion medium, for example, it has been observed that the suspension of talc in water is improved by the addition of the finely ground magnesium carbonate containing mineral, Ultracarb US (LKAB Minerals). However, the addition of a secondary particulate material may add cost to the formulation and has not been found to be necessary in the preferred embodiments provided that the coating mixture is kept agitated, for example, by stirring, As regards the adhesion promoter, this may be selected from a wide range of water based adhesives, either on their own or in combination. Examples of such adhesives include natural products, such as starch, cellulose, gelatine and natural rubber (INR) latex, as well as synthetic latexes and water soluble polymers, such as polyvinyl acetate (PVA) and polyviny' alcohol (PVOH), and emulsions, although other adhesives are also contemplated. Careful investigation has shown that in many instances the somewhat conflicting requirements of good adhesion and ease of removal cannot be achieved by a single adhesive, or type of adhesives, and a combination of more than one adhesion promoter is required to meet these requirements.

In the preferred embodiments the adhesion promoter is a combination of two cellulose derivatives, sodium carboxymethylcellulose (Blanose 7L, Ashland) and cellulose ether (Methocel F4C, Dow Chemical Company). These cellulose derivatives have the advantage of giving a coating which is easily applied and gives a uniform coating that is easily washed off However, in some instances, the adhesion of a coating based solely on these cellulose derivatives is not adequate and the coating has a tendency to flake off, particularly in the gas absorption stage where the polymer artefact and its coating are exposed to high pressure and elevated temperature. In such instances, as demonstrated in one of the preferred embodiments, a styrene acrylic copolymer emulsion (Texicryl 13-074, Scott Bader) may also be added to improve adhesion and increase the flexibility of the coating, making it less friable while still allowing the coating to be removed at the end of the process by washing with water.

As regards wetting agents, these may be added to improve the ability of the coating mixture to wet the surface of the polymer artefact when the coating is applied. The wetting agent may be selected from a wide range of commercially available products typically designed to reduce the surface tension of a liquid. However, as a result of reducing the surface tension, the addition of a wetting agent may also increase the propensity for the coating mixture to form a froth and so the addition of an antifoaming agent may also be necessary.

As regards antifoaming agents, these may be added to prevent the generation of excessive amounts of froth either in the mixing tanks of the coating equipment or during the washing process. The antifoaming agent may be selected from a wide range of commercially available products typically supplied for such applications, however, it is preferable if the antifoaming agent does not contain silicone as residues of the antifoaming agent may affect the ability to bond the final polymer foam using adhesives, No antifoaming agent is used in the preferred embodiment, although it has been found to be advantageous to add an antifoaming agent to the water used to wash the coating mixture from the polymer foam at the end of the process to prevent excessive frothing.

EXAMPLE I

By careful development, the following coating formulation has been found to provide the required level of adhesion and washability for sheets of polyolefin, while preventing the polymer artefact sticking to the tray on which it is placed during the gas absorption and expansion stages of the process. By limiting the number of components in the coating formulation it is relatively simple to make and use and the formulation costs are kept to a minimum.

S

Coating Formulation Water 100 kg Talc S2 (LKAB Minerals) 50 kg Blanose 7L (Ashland) 1.5 kg Methocel F4C (Dow Chemical Company) 0.6 kg The coating formulation is applied to the surfaces of the polymer artefact, in this case a thick LDPE (low density polyethylene) sheet, by passing the sheet under a series of spray nozzles until an even coating is achieved. First one side of the sheet is coated and the coating dried under hot air before the other side of the sheet is coated and the coating again dried under hot air, Once both of the coated surfaces are dry, the sheet is placed on a tray in a high pressure autoclave for the gas absorption stage of the process.

In this example the LDPE sheet is exposed to a temperature of 250°C and a pressure of 670 bar of nitrogen until the LDPE is fully saturated with gas. Once the gas absorption stage has been completed the sheet is first cooled and then removed from the high pressure autoclave, Some minor cracking of the coating can be observed as a result of the expansion of the sheet during the gas absorption process but the coating is essentially intact and the gas containing sheet can be placed on a tray in the lower pressure expansion autoclave without any remedial treatment to the coating. In the lower pressure autoclave the gas containing sheet is first heated under pressure then expanded to the final foam dimensions, The foamed sheet is then removed from the low pressure autoclave and allowed to cool before the coating is removed by washing the surfaces of the sheet with water.

Failure to coat the sheets properly will result in the LDPE sheets sticking to the trays used in the high pressure autoclave for the gas absorption stage of the process. As the

S

trays are arranged in closely packed stacks (tray sets), to maximise the loading efficiency of the autoclave, the sheet will stick to the tray on which it has been placed and, in certain circumstances, may stick to the tray above, it for example, the sheet exhibits a slight bow. Sheets which stick to the trays will typically show some form of distortion which would make them unsalable and so they are scrapped at this stage in the process. In addition to the cost of scrapping of the sheets, sticking of the sheets can also cause damage to the tray sets resulting in increased maintenance and labour costs.

Where the sheet is not coated properly and has, for example, bare patches, the sheet may show no evidence of having stuck to the tray during the gas absorption stage of the process, however, after expansion in the low pressure autoclave the expanded foam sheet will typically exhibit an area of inhomogeneity. This is typically evidenced by a visible patch on the surface of the expanded sheet which is accompanied by an area of coarse and irregular cell stmcture below the surface. Such an area may penetrate deep within the sheet resulting in a loss of yield or, in more severe cases, the scrapping of the entire sheet.

Another consequence of a sheet not having been coated properly in the first instance, or the coating flaking off during previous stages in the process, is the failure of the sheet to expand uniformly in the low pressure expansion stage. This may result in expanded sheets which do not have a regular rectangular shape, or where the corners of the sheets have become folded under as a result of the sheet sticking to the tray in the lower pressure autoclave either before or during expansion. After expansion and before the expanded sheet has had time too cool it may still stick to the tray of the lower pressure autoclave if there are gaps in the coating. Any distortion in the final sheets may result in loss of yield or scrapping of the entire sheet. As with the high pressure autoclave stage, in addition to the yield loss or the cost of scrapping of the sheets, sticking of the sheets can also cause damage to the lower pressure expansion tray sets resulting in increased maintenance and labour costs.

EXAMPLE 2

Further careful development has shown that the coating formulation developed for sheets of polyolefin is not suitable for all materials, and in particular sheets of polyvinylidene fluoride (PVDF). While preventing the polymer artefact sticking to the tray on which it is placed during the gas absorption and expansion stages of the process, the presence of talc in the formulation results in an acceleration of the thermal degradation of the polyvinylidene fluoride resin which is manifest as a colour change in the foam. Depending on the duration of the gas absorption process this colour change can range from a light pink colour to a dark purple colour and, in the most severe cases, may even result in a black glassy layer being formed on the surface of the foam. In such cases, the foam does not expand uniformly and the result is a distorted sheet.

Colour changes and distortion can both result in loss of yield or scrapping of the entire sheet. However, by replacing talc with the ground magnesium carbonate containing mineral, Ultracarb US, and adding a styrene acrylic copolymer emulsion (Texicryl 13- 074, Scott Bader) to improve the adhesion, it is possiNe to produce a coating with all the attributes of the coating described in Example I but without the issues relating to PVDF degradation.

Coating Formulation Water 50 kg Blanose 7L (Ashland) 0.08 kg Methocel F4C (Dow Chemical Company) 0.03 kg Texicrvl 13-074 (Scott Bader) 3 kg Ultracarb US (LKAB Minerals) 15 kg

Claims

CLAIMS1 A coating formulation for application to a polymer artefact to reduce adhesion of the artefact to a surface, wherein the coating formulation comprises at least one of each of the following constituents: (i) a dispersion medium; (ii) a particulate material; and (iii) an adhesion promoter. I0
2. A coating formulation as claimed in claim, additionally comprising a wetting agent and/or an anti-foaming agent.
3. A coating formulation as claimed in claim 2 or 3, wherein the dispersion mediumiswater.
4. A coating formulation as claimed in any preceding claim, wherein the particulate material has a lamellar or plate-like structure.
5. A coating formulation as claimed in any preceding claim, wherein the particulate material is thermally stable at 250°C.
6. A coating formulation as claimed in any preceding claim, wherein the particulate material is a mineral.
7. A coating formulation as claimed in claim 6, wherein the particulate material is talc, a magnesium carbonate containing mineral, or a combination thereof
8. A coating formulation as claimed in any preceding claim, wherein the adhesion promoter is a water based adhesive or combination of water based adhesives.
9. A coating formulation as claimed in claim 8, wherein the adhesion promoter is a combination of cellulose derivatives.
10, A coating formulation as claimed in claim 9, wherein the adhesion promoter is a combination of cellulose derivatives and polyvinyl acetate (PVA).11 A coating formulation as claimed in any one of claims 9 or 10, wherein the cellulose derivatives comprise a sodium carboxymethylcellulose and a cellulose ether.12. A coating formulation as claimed in any preceding claim, comprising the following constituents: (i) a dispersion medium which is water; (ii) a particulate material which is either talc or a magnesium carbonate containing mineral; and (iii) an adhesion promoter which is a sodium carboxymethylcellulose, a cellu'ose ether, a styrene acrylic copolymer emulsion, or any combination thereof 13. A coating formulation substantially as hereinbefore described with reference tothe examples.14. A polymer artefact having a coating formulation as claimed in any preceding daim applied thereto.15. A process for the application of a coating formulation as claimed in any of daims Ito 13, wherein the coating formulation is applied to the surface of a polymer artefact by spraying or brushing.16. A process for the removal of a coating formulation as claimed in any of claims I to 13, wherein the coating formulation is removed from the surface of a polymer artefact by washing with water.Amendments to the claims have been filed as follows:CLAIMS1. A coating formulation for application to a polymer artefact to reduce adhesion of the artefact to a surface, wherein the coating formulation comprises at least one of each of the following constituents: U) a dispersion medium which is water; (ii) a particulate material which is either talc or a magnesium carbonate containing mineral; and (iii) an adhesion promoter which is a combination of a sodium carboxymethylcellulose and a cellulose ether.2. A coating formulation substantially as hereinbefore described with reference to IC)the examples.o 3. A polymer artefact having a coating formulation as claimed in any preceding QU) claim applied thereto.4. A process for the application of a coating formulation as claimed in any of claims 1 or 2. wherein the coating formulation is applied to the surface of a polymer artefact by spraying or brushing.5. A process for the removal of a coating formulation as claimed in any of claims 1 or 2, wherein the coating formulation is removed from the surface of a polymer artefact by washing with water.